This invention is directed to a printer and, in particular, to a small-sized serial type thermal printer in which a thermal printing head is laterally driven across a thermosensitive recording medium to carry out printing.
In conventional serial-type thermal printers, printing of characters is performed by serially pressing a thermal printing head to a thermosensitive recording medium or paper against a platen and then actuating the printing head. In such printers, the pressure of the thermal printing head against the platen must be relaxed in order to allow for advancement of the thermosensitive paper past the printing head and to allow for lateral translation of the thermal printing head across the thermosensitive paper. Therefore, printers have been constructed as full cycle machines wherein the printing head is required to move laterally across the full width of the paper before return is possible. For example, a rotatable cylinder having a helical drive surface has been provided for reciprocating the printing head across the paper as described in U.S. Pat. No. 3,986,594. Another type of full cycle mechanism is described in U.S. Pat. No. 4,250,808.
However, such printers have the following disadvantages: (1) since the printing mechanisms are full cycle machines, even if a few characters are to be printed along one line, the printing head must traverse the entire width of the printing medium before returning to a reset position; (2) it is difficult to increase the speed of operation since the printing head must fully traverse the recording medium; (3) electrical power is wastefully consumed; (4) the useful life of the printing head is shortened due to excess abrasion against the printing paper and the platen; and (5) printing paper feeding at high speed is not possible.
Prior printer constructions which attempted to provide for return of the printing head to the reset position from any printing position require additional power sources or intricate mechanisms for paper feeding and platen releasing. This prevents both miniaturization of a printer and reduction in the manufacturing costs.
Attempts have been made to mechanically couple the paper feeding mechanism in a printer to the main driving shaft which causes the printing head to move across the recording paper. Such prior constructions are noisy in operation since a toothed lever connected to the paper feeding roller in the paper feeding mechanism crashes against a toothed wheel coupled to the main driving shaft. In addition, since it has proven difficult to obtain a given amount of rotation in either direction on the same axis as the main driving shaft which itself can rotate by any amount in either direction, the miniaturization of a printer is difficult.
Accordingly, a small-sized serial-type printer in which the printing head drive mechanism, paper feeding mechanism and platen releasing mechanism are operatively coupled and which has a simplified construction and is relatively inexpensive to manufacture, is desired.